Health experts are to hold an emergency meeting in Johannesburg this week, following the discovery of a deadly new strain of tuberculosis.The strain - known as extreme drug-resistant TB - has horrified World Health Organisation doctors. In one outbreak in South Africa, 52 of 53 patients died within weeks of becoming infected.

'This new strain leaves us facing a nightmare,' said Paul Nunn, coordinator of the WHO's drug-resistance unit. 'It is resistant to nearly every drug in our arsenal. We are now on the threshold of the appearance of a strain of TB that is resistant to every medicine known to science.'

Deadliest strain brought in by visitors from other countries, officials say

SAN FRANCISCO - The worst forms of the killer tuberculosis bug have been gaining ground in the United States, alarming public health officials over imported drug-resistant strains of a disease that is mostly under control in this country.

Although the number of drug-resistant TB cases in the U.S. is small compared to developing nations, health officials here warn that visitors from other countries who are unaware of their infections are bringing over the deadliest mutations.

Often those with drug-resistant strains stop taking their medicine when they feel better but aren’t cured.

That’s what happened with Pich Chhieng, 61, a teacher who was infected in his native Cambodia and carried it with him to this country. He took medication for eight months but abruptly stopped because he ran out of money and was feeling much better.

He didn’t know until he was hospitalized while visiting family in Los Angeles that by neglecting his treatment he had allowed the disease to mutate, and the drug-resistant bacteria had overwhelmed his lungs.